The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2001-043596 filed on Feb. 20, 2001 the entire contents thereof is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a decompression unit for reducing compression pressure for facilitating startup when starting a reciprocating internal combustion engine.
2. Description of Background Art
Hithertofore, a decompression unit for internal combustion engines is disclosed in Japanese Utility Model Registration No. 2534274. The decompression unit is provided in an engine having an air intake rocker arm for opening two air intake valves respectively with its bifurcated extremities and includes a decompression arm inserted between one of the air inlet valves and one of the extremities of the air intake rocker arm. A decompression cam is provided for pivoting the decompression arm. At the time of startup, the decompression cam pivots the decompression arm to push one of the air intake valves to open for reducing the compression pressure. Accordingly, a reaction force of the valve spring of the air intake valve to be applied to the decompression arm and the decompression cam is applied only by one of the air intake valves and thus is reduced by half, whereby durability of the decompression cam increases and the operating force may be reduced.
In the related art, the decompression arm is inserted between the air intake rocker arm and the air intake valve. Therefore, even after the decompressing operation in which the air intake valve is opened by the decompression arm at the time of startup is released, the air intake cam always pivots the decompression arm as well as the air intake rocker arm when the air intake valve is opened and closed by the air intake cam. As a consequent, the equivalent inertia weight of the valve driving system for opening and closing the air intake valve increases by the weight corresponding to the decompression arm, and thus the followability in the opening-and-closing operation of the air intake valve with respect to the air intake cam while the internal combustion engine is operated at high-revolution speeds decreases, which results in a decrease in output of the engine. Since the decompression arm is always pivoted together with the air intake rocker arm, the decompression arm is required to have a rigidity for bearing with a pivotal motion when the internal combustion engine is operated at high-revolution speeds after startup, and to have an abrasion resistance for limiting abrasion caused by contact with the air intake rocker arm and with the air intake valve in a state in which a pressing force from the air intake cam and a reaction force of the valve spring are applied thereto, which results in a disadvantageous problem in that the cost increases.
In order to solve the problems described above, the present invention is directed in common to provide a decompression unit for internal combustion engines wherein a decompression unit including a decompression arm provides good followability in the opening-and-closing operation of the air intake valve or the exhaust valve with respect to the cam at the time of high-velocity revolution is satisfactory without increasing the equivalent inertia weight of the valve driving system, and the cost can be lowered. The present invention provides a compact decompression unit in which the decompressing operation can be performed manually even when the decompressing operation is released by automatic operation. The present invention is directed to provide a decompression unit in which the optimal decompressing operation can be performed by adjusting the valve opening period and the lifting amount.
The present invention is directed to an internal combustion engine including an air intake valve and an exhaust valve wherein the rocker arm that is pivoted by a cam provided on the cam shaft that is driven by the power of the crankshaft opens and closes the air intake valve or the exhaust valve, a decompression unit for an internal combustion engine comprising a first decompression cam that takes the operating position when the number of revolutions of the engine is not more than the predetermined number of revolutions during startup. A decompression arm is supported by the internal combustion engine for a pivotal motion and includes a first abutting portion for abutting against the first decompression cam and a pressing portion for pressing the rocker arm. A resilient member applies a resilient force so that the first abutting portion of the decompression arm is brought into abutment against the first decompression cam, wherein the decompression arm is pivoted by the first decompression cam located at the operating position against the resilient force for allowing the rocker arm to pivot via the pressing portion for opening the air intake valve or the exhaust valve.
According to the present invention, since the resiliency of the resilient member acts on the decompression arm so that the decompression arm moves away from the rocker arm, the decompression arm is pivoted together with the rocker arm for pivoting the rocker arm only when it is pivoted by the first decompression cam located at the operating position, but the decompression arm is not pivoted together with the rocker arm when the rocker arm is pivoted by the cam. Therefore, the following effects are achieved. Since the decompression arm is pivoted with the rocker arm only when it is pivoted by the decompression cam during the decompressing operation, but is not pivoted when the rocker arm is pivoted by the cam and when the decompressing operation is released, the provision of the decompression arm does not increase the equivalent inertia weight of the valve operation system for opening and closing the air intake valve or the exhaust valve, and thus the followability of the air intake valve and the exhaust valve in the opening-and-closing operation with respect to the cam is satisfactory when the internal combustion engine E is operated at high-revolution speeds, thereby preventing a lowering of the output of the engine. Since the number of revolutions of the engine at which the decompression arm is pivoted is not more than the predetermined number of revolutions during startup, which belongs to the extremely low revolution range in the operational revolution range of the internal combustion engine, the requirements for the rigidity of the decompression arm are not strict. In addition, since abrasion caused by contact with the rocker arm is seen only during the decompressing operation, the requirements for abrasion resistance are also not strict. Therefore, the cost of the decompression unit can be reduced by using less expensive materials or by omitting the surface treatment while ensuring the durability.
The present invention includes a decompression unit for an internal combustion engine wherein the decompression unit comprises a second decompression cam that takes the operating position by a manual operation. The decompression arm includes the second abutting portion that abuts against the second decompression cam, and the decompression arm pivoted against the resilient force by the above-described second decompression cam located at operating position pivots the rocker arm via the pressing portion to open the air intake valve or the exhaust valve.
According to the present invention the decompressing operation can be performed by the second decompression cam that is to be operated manually even in the operational revolution range in which the decompressing operation of the first decompression cam is released. Therefore, the following effects are achieved in addition to the effects of the present invention, the decompressing operation can be performed manually even when the automatic decompressing operation based on the number of revolutions of the engine is released. Accordingly, when a state in which an unburned air-fuel mixture exists in the combustion chamber occurs, for example, if an accidental fire due to a concentrated air-fuel mixture supplied during startup occurs when the internal combustion engine in a state in which the decompressing operation by the first decompression cam is released is operated, the unburned air-fuel mixture can be scavenged quickly by the decompressing operation via the second decompression cam so that the normal operating state such as a restart or the like is restored. In addition, since the decompression arm can be used in common in the automatic operation and the manual operation, the decompression unit can be downsized.
The present invention provides a pressing portion comprises an adjusting member being capable of adjusting the space with respect to the abutting portion of the rocker arm that abuts against the pressing portion.
According to the present invention, since the space between the pressing portion of the decompression arm and the abutting portion of the rocker arm can be adjusted, the opening-and-closing timings, the valve opening periods, and the lifting amounts of the air intake valve or the exhaust valve by the decompression arm can easily be adjusted at the time of the decompressing operation. Therefore, the optimal decompressing operation can be performed for each internal combustion engine, and the decompression unit can be used in common for a variety of internal combustion engines, which enables a cost reduction by mass production of the decompression unit.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.